In the art of making paper pulp from comminuted cellulosic fibrous material, such as wood chips, utilizing a continuous digester, there is a continuing effort to improve the quality of the chemical pulp produced. For example in co-pending application Ser. No. 07/200,204 filed May 31, 1988, a method and apparatus are disclosed for enhancing the quality of chemical pulp by minimizing the mechanical action on the chips, especially when high temperature and pressure conditions exist or chemicals are present. Also that application teaches procedures for making the treatment more uniform. According to the present invention, it is again desired to even further increase the quality of the chemical pulp produced.
According to the present invention, the quality of the chemical pulp--particularly its tear strength and desired beating properties--is enhanced in a simple manner. In conventional systems, withdrawn black liquor is passed to a series of flash tanks to be concentrated, and then is passed to evaporators for ultimate recovery of the chemicals thereof. According to the present invention, the black liquor from the first flash tank is passed to a mechanism or earlier process step in which the comminuted cellulosic fibrous material is being slurried with a liquid. In this way, the material, such as wood chips, remains in contact with the black liquor for a significant period of time, the black liquor partly sulfonizing the lignin in the chips. Ultimately, the slurry is passed to a high pressure feeder, and then to a solids/liquid separator, to separate out the black liquor to return it for ultimate recovery (including utilizing a second flash tank), whereupon white liquor or the like is added before the material is actually fed to the operative portions of a continuous digester for a chemical cook thereof at higher sulfidity than that of the white liquor added.
According to the present invention there is provided a method of continuously digesting comminuted cellulosic fibrous material, with increased sulfidity, to produce paper pulp utilizing a continuous digester. The method comprises the steps of: (a) Steaming the comminuted cellulosic fibrous material. (b) Slurrying the material with liquid. (c) Feeding the liquid slurry of steamed material to the digester. (d) Effecting kraft cooking of the material in the digester utilizing white liquor. (e) Withdrawing black liquor from the digester. (f) Recirculating the black liquor to the steamed comminuted cellulosic fibrous material in step (b) to provide at least part of the slurrying liquid. (g) Replacing the majority of the black liquor of the slurry with white liquor or the like prior to step (d), and separating out the replaced black liquor. And, (h) passing the separated out black liquor to recovery or disposal. Step (f) is preferably practiced by passing the black liquor to a first flash tank to produce steam and concentrated black liquor, and step (a) is practiced in part by utilizing the steam from the first flash tank.
According to another aspect of the present invention, there is provided apparatus for continuously digesting cellulosic fibrous material to produce chemical pulp. The apparatus comprises: (a) Steaming means for steaming the material. (b) Liquid slurrying means for producing a liquid slurry containing the steamed material. (c) A high pressure feeder having a low pressure inlet, low pressure outlet, high pressure inlet, and high pressure outlet. (d) A continuous digester. (e) Black liquor withdrawal means for withdrawing black liquor from the continuous digester. (f) A first flash tank containing steam and concentrated liquor outlets. (g) Connecting means for interconnecting the concentrated liquor outlet from the first flash tank to the slurrying means. (h) Black liquor recovery or disposal means. And, (i) means for operatively connecting the low pressure outlet of the high pressure feeder to the recovery or disposal means. The placement of the high pressure feeder and a solids/liquid separator depends upon the particular digester utilized. Digesters such as shown in said co-pending application Ser. No. 07/200,204 filed May 31, 1988, or in co-pending application Ser. No. 007/372,650 filed June 24, 1989 entitled "Continuous Cooking with Reduced Cost, Horsepower, and Pulp Degradation" or conventional continuous digesters, may be utilized. Slurrying of the material can take place in a chute disposed downstream of a horizontal steaming vessel, or in a vertical vessel having steaming at the top and a liquid level therein.
It is the primary object of the present invention to provide for the production of chemical pulp utilizing a continuous digester with enhanced quality as a result of increased sulfidity. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.